FIV infection has been attracting attention as one of the most important infectious diseases in the area of small animal veterinary medicine, and also as an animal model for human immunodeficiency virus infection. Since the discovery of FIV in 1986, studies on FIV vaccines were conducted worldwide, but vaccine development proved to be difficult as FIV has multiple subtypes. Dr. Yamamoto of the United States successfully developed a highly effective vaccine by combining multiple subtypes, which was named the multi-subtype vaccine (Non-patent Document 1). Use of the FIV Shizuoka strain, which was discovered in a collaborative research between Kitasato University and the Kitasato Research Institute, as the antigen for this multi-subtype vaccine was effective for increasing the efficacy (Non-patent Document 2). This multi-subtype vaccine containing the FIV Shizuoka strain was marketed in 2002 by Fort Dodge Animal Health of the U.S. (Product name; Fel-O-Vax FIV). This vaccine drew attention as the world's first lentivirus vaccine.
However, vaccination with this FIV vaccine led a new problem concerning diagnostic methods for FIV infection. The issue concerns how to distinguish FIV-infected cats from FIV-vaccinated cats. Until then, FIV infection was diagnosed by detecting anti-FIV antibodies. Diagnostic agents for detecting anti-FIV antibodies are commercially available from several companies, and are commonly used for clinical diagnoses. However, cats vaccinated with the FIV vaccine would also be judged positive with these diagnostic agents as such cats will also produce anti-FIV antibodies. That is, with the current diagnostic agents, both FIV-infected cats and FIV-vaccinated cats will be deemed positive, and it cannot be distinguished whether an anti-FIV antibody-positive cat is actually an FIC-infected cat. As a result, FIV vaccination relatively decreases the significance of such serological tests. This issue has become a limiting factor for the widespread use of the FIV vaccine.
As a method for diagnosing FIV infection, test methods for detecting viruses are also available. Test methods for detecting viruses do not depend on the presence or absence of antibodies; therefore, they allow correct diagnosis independently of vaccination. Generally-used, practical methods for detecting viruses are gene amplification methods represented by the PCR method. Since the PCR method is a sensitive and highly specific method, its diagnostic significance is high, and is an effective test method when a diagnosis cannot be confirmed by a serological test method. However, since special materials and equipments are necessary for these gene amplification methods, they are not suitable for use in clinical practice. Consequently, convenient test methods for distinguishing FIV infection and FIV vaccination that can also be used in clinically are required.    [Non-patent Document 1] Pu R, Coleman J, Omori M, Arai M, Hohdatsu T, Huang C, Tanabe T, Yamamoto J K., Dual-subtype FIV vaccine protects cats against in vivo swarms of both homologous and heterologous subtype FIV isolates. (2001) AIDS 15 (10): 1225-1237.    [Non-patent Document 2] Hohdatsu T, Okada S, Motokawa K, Aizawa C, Yamamoto J K, Koyama H., Effect of dual-subtype vaccine against feline immunodeficiency virus infection. (1997) Vet. Microbiol. 58 (2-4): 155-165